Auto-inhibition at a ligand-gated ion channel: a cross-talk between orthosteric and allosteric sites.

BACKGROUND AND
PURPOSE: A ligand is believed to produce either positive or negative responses, or to block both of them. However, an indole compound was found to promote both positive and negative effects at the 5-HT3 AB receptor, which displays a low level of spontaneous activity. The present study attempted to delineate the mechanisms underlying this phenomenon. EXPERIMENTAL APPROACH: The spontaneously active V291S 5-HT3 A receptor was used to explore the properties of 5-hydroxyindole (5-HoI) and 5-methoxyindole (5-MoI), structural analogues of 5-HT, either alone or in combination with orthosteric probes. KEY
RESULTS: Two types of efficacy switching were initiated by altering ligand structure and concentration. At lower concentrations, a subtle structural change at position 5 of the indole molecule resulted in opposite effects. 5-HoI apparently elicited partial allosteric inverse agonism, whereas 5-MoI induced allosteric agonism. Interestingly, at a higher concentration, these indoles produced distinct auto-inhibition, manifested as a switch from positive to negative effects. 5-HoI induced a transition from orthosteric agonism to allosteric inverse agonism, whereas 5-MoI produced a shift from allosteric agonism to orthosteric inverse agonism. The auto-inhibition appears to involve communication between orthosteric and allosteric sites of the active receptor conformation and/or between inactive and active conformations. An additive effect of orthosteric and allosteric inverse agonism and insensitivity of allosteric agonism to orthosteric antagonism were also demonstrated. CONCLUSIONS AND IMPLICATIONS: Together, the results suggest that the moiety at position 5 of the indole structure is a critical determinant of a ligand's properties at the 5-HT3 A receptor, providing new insights into understanding ligand-receptor interactions.